1. Field of the Invention
The present invention relates to a bidirectional switch and a method for driving the bidirectional switch.
2. Description of the Prior Art
As power switching semiconductor devices, a power MOSFET (metal oxide film semiconductor field-effect transistor), an IGBT (insulated-gate bipolar transistor), a thyristor and the like are known. When a switching circuit conducting a bidirectional current using such semiconductor devices is formed, each of the semiconductor devices has to have a bidirectional breakdown voltage. To have a bidirectional breakdown voltage means to have a breakdown voltage with respect to both positive and negative voltages.
A power MOSFET and an IGBT each have, in general, a low reverse blocking capability. Therefore, for example, to realize a bidirectional switch using an IGBT, as shown in FIG. 13, it is necessary to connect two IGBTs in parallel and in a direction opposite to each other and connect diodes to the IGBTs in series, respectively. In FIG. 13, an IGBT 201 and a diode 202 are connected to each other in an opposite direction to the direction in which an IGBT 203 and a diode 204 are connected. Thus, by turning the IGBT 201 and the IGBT 203 both ON, a current flows bidirectionally, and by turning the IGBT 201 and the IGBT 203 both OFF, a high breakdown voltage for both polarities can be achieved.
In a semiconductor device for performing such bidirectional switching, it is important to reduce switching loss determined according to a product of transitional voltage and current generated at a time of switching and conduction loss resulting from power consumption by a resistance (referred to as an “ON resistance”) of the semiconductor device in an ON state. However, it is difficult to reduce an ON resistance of a bidirectional switching circuit formed using a semiconductor device made of silicon (Si) because of material limits of Si.
To overcome material limits and reduce conduction loss, introduction of a semiconductor device using a wide-band-gap semiconductor such as nitride semiconductor represented by GaN, silicon carbide (SiC) or the like has been examined. Wide-band-gap semiconductor has a breakdown electric field which is higher by approximately 1 digit order, compared to Si. Specifically, due to spontaneous polarization and piezopolarization, charges are generated at a hetero junction interface of aluminum gallium nitride (AlGaN) and gallium nitride (GaN). Thus, even in an undoped state, a two-dimensional electron gas (2DEG) layer having a high sheet carrier concentration of 1×1013 cm−2 or more and a high mobility of 1000 cm2V/sec or more is formed. Therefore, an AlGaN/GaN hetero junction field-effect transistor (AlGaN/GaN-HFET) is expected as a power switching transistor for realizing low ON resistance and high breakdown voltage.
However, in a regular FET, a breakdown voltage between gate and source is lower than a breakdown voltage between gate and drain. Therefore, even a FET using wide-band-gap semiconductor needs two FETs and two protective diodes to realize a bidirectional switch.
To equalize a breakdown voltage between gate and source voltage with a breakdown voltage between gate and drain, a distance between a gate electrode and a source electrode can be equalized with a distance between the gate electrode and a drain electrode. Use of a FET in which a breakdown voltage between gate and drain is equalized with a breakdown voltage between gate and source in a bidirectional switching circuit in the above manner has been proposed (see, for example, Specification of U.S. Patent Application No. 2005/0189561).
However, even when a breakdown voltage between gate and drain is equalized with a breakdown voltage between gate and source, a high breakdown voltage of a bidirectional switch can not be realized. In a typical FET, a current flowing from a drain electrode to a source electrode is controlled by applying a voltage between a gate electrode and the source electrode. But even though a voltage is applied between the gate electrode and the source electrode, a current flowing from the source electrode to the drain electrode can not be controlled. Therefore, a bidirectional switch in which a current flowing bidirectionally between the source electrode and the drain electrode has to be controlled can not be realized when only one FET is provided.